1. Field of the Invention
The present invention relates to a method and an apparatus for producing an optical fiber. More particularly, the present invention relates to improvement of a method and an apparatus for producing an optical fiber which comprises heating and melting a glass preform for an optical fiber (hereinafter referred to as "preform"), drawing the melted preform from its one end at a constant tension to form a fiber and coating a peripheral surface of the drawn fiber with a resin to finish an optical fiber.
2. Description of the Related Art
Usually, an optical fiber is produced as follows:
As shown in FIG. 1, a preform 1 produced in a separated step is inserted in a muffle tube 3 of a heating furnace 2 and the melted glass is drawn from its lower end at a constant tension. A drawn fiber 4 is cooled by spontaneous heat radiation during drawing and then passed through a coating die 5 containing a liquid resin to coat the resin around a peripheral surface of the drawn fiber 4. Thereafter, the resin is hardened in a furnace 9 and a finished optical fiber 7 is wound by a drum 8.
The reasons why the peripheral surface of the drawn fiber 4 is coated with the resin during the production of the optical fiber 7 are as follows:
Since the drawn fiber 4 is a very thin and fragile glass fiber and has a diameter of 100 to 150 .mu.m, it is so weak as to be broken under a tensile load of about 100 g.
Since the drawn fiber is made of glass, different from metals, it tends to suffer from brittle facture and a minute flaw made on the fiber surface easily grows and finally breaks the optical fiber.
Therefore, the surface of the drawn fiber is coated with the resin to increase tensile strength and flexural strength.
To increase productivity of the optical fiber, a drawing rate of the fiber should be increased. When the peripheral surface of the drawn fiber is coated with a synthetic resin such as an ultraviolet curable resin, since the drawn fiber is drawn from the melt preform, sometimes it is not sufficiently cooled although it is cooled by radiation during drawing. If the insufficiently cooled drawn fiber is passed through the coating die 5, a coated layer of the resin is thermally affected by the drawn fiber and a viscosity of the resin around the drawn fiber is decreased too much so that a thickness of the coated layer is not uniform.
To overcome such drawbacks, conventionally, a gas is blown against the drawn fiber just after it is drawn from the melt preform (cf. Japanese Patent Publication No. 39496/1979). This method is shown in FIG. 2, in which the reference 6 represents a cooling gas, and the same references represent the same parts as in FIG. 1.
As the drawing rate is increased to improve the productivity, an amount of heat to be deprived from the drawn fiber should be increased. When the cooling gas is blown in a direction parallel to the fiber as in the method shown in FIG. 2, a gas near the fiber is hardly exchanged and the heat is transferred from the gas near the fiber to the gas in an outside layer by heat conduction. Therefore, the cooling efficiency in this method is not satisfactory. To increase the heat conduction, helium or hydrogen can be used. However the former is expensive and the latter is dangerous due to explosion.